The present invention relates generally to the operation of an electrolytic cell, and particularly to a method that permits continuous monitoring of leakage current between electrical leads of the cell and an outer metal shell of the cell.
Electrolytic cells having a corrosive environment are generally provided with an interior lining of corrosion and heat resistant, insulating materials, these materials being contained in an outer structural shell of a material such as steel. In the operation of such cells there is always the danger that constituents of the corrosive environment will enter cracks, breaks or pores of the insulating materials, and thereby reach the metal shell, as explained, for example, in U.S. Pat. Nos. 3,773,643 and 3,779,699 to Russell and Knapp. Upon reaching the shell, the constituents corrode and eventually eat through the shell. Incipient conditions of this kind can be determined by measuring DC current between the shell and the bus or lead supplying current to or removing current from the cell, for example as shown in U.S. Pat. No. 3,406,103 issued in the name of Carl W. Raetzsch. However, continuous DC measurements have certain detrimental effects in the operation of the cell. In the case of cells designed to produce aluminum by electrolysis in a bath of molten salt, continuously monitoring DC current between the shell and a current carrying conductor connected to the cell causes stringers or fingers of aluminum metal to grow inwardly from the shell if the shell is made cathodic by the monitoring process and apparatus, and outwardly from the cathode if the shell is anodic. This, of course, increases leakage current, as the metal of the stringers is much more effective in conducting electrical current than the corrosive constituents creating the original problem. In order to avoid such stringer growth, only periodic DC measurements can be taken, with the time between these measurements affording opportunity for further corrosive attack on the shell to go unnoticed and unmonitored.
As further indicated in the above Russell and Knapp patents, if the cell shell becomes anodic nascent chlorine is produced on the surface of the shell which quickly eats away and perforates the shell. Thus, a DC connection that renders the shell anodic is to be avoided also.
Another disadvantage in using DC measurements to determine the existence of fluid leaks in a cell lining is that the accuracy of such DC measurements is adversely affected by the electrochemical phenomenon of electrolysis that occurs within the operating cell. This is a DC phenomenon which is generally not constant, as conditions within the cell change due to the continuous decomposition of the materials employed in the process, which are in turn being continually replenished by additions of the materials to the cell. Such changing conditions will be reflected in any DC measurement taken between the shell and the cell bus.